Basic Types of Bipolar Transistor

The alloyed-junction (or fused-junction) transistors. In sketch form, their struc­ture is shown in Fig. 4.24. It is essentially an n -type semiconductor wafer, with two dots containing p -type impurities alloyed into the opposite sides of the wafer to provide an emitter and a collector junction. Since the collector junction is to dissipate more power, it is usually made larger in size than the emitter junction.

Fig. 4.24. Structure of an alloyed transistor

The best among diffused transistors are pla­nar transistors. In them the p-n junctions are formed by the diffusion of impurities through an opening in a protective coat applied to the surface of a semiconductor so that the emitter, base and collector regions come all to the same plane surface, that is, terminate in the same geometrical plane-hence the name 'planar'.

Fig. 4.25. Structure of a planar transistor

Epitaxial-planar transis­tors are a further step forward in the planar technology. In the fabrication of such transistors, a thin layer of, say, n -type semiconductor which has a high resistivity is deposited onto a low-resistivity collector n⁺- sub­strate of the same type of conduction, following which the p- base and n⁺- emitter regions are produced by the double diffusion technology (Fig. 4.26).

Fig. 4.26. Structure of a planar-epitaxial transistor

Bipolar microwave transistors ordinarily use contacts (metallization) configured so that the emitter region consists of a number of narrow strips or small dots. This serves to reduce the series base resistance and the detrimental 'cur­rent crowding' effect-an outward bulging of electric flux lines at the edges. The metallization geometry is important for high-power transistors because the 'current crowding' effect is particularly strong at heavy currents.

Fig. 4.27. Electrode structure of high-power microwave transistors: (a) interdigitated; (b) overlay. (1) Base lead; (2) emitter region; (3) emitter lead

High-power microwave transistors can deliver a pulse power of as much as 100 W at frequencies as high as 1 GHz, and 5 to 10 W at 4 or 5 GHz and higher.

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